Initiation of the Western Interior foreland basin

Abstract

In this issue of Geology, Fuentes et al. (2009, p. 379) present a new subsidence plot and age data from detrital zircons that help to constrain the date of initiation of the foreland basin in Montana (United States). They offer these data as evidence for the resolution of a controversy regarding the initiation of foreland basin sedimentation in the Western Interior. However, their discussion raises the question of what geological data constitute the defi nitive evidence of the foreland basin style of sedimentation, and therefore the criteria that should be used to defi ne its commencement. A useful defi nition of a foreland basin is that it is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric fl exure. At convergent plate margins, foreland basins develop behind the marginal arc (retroarc or retroforeland type; Jordan, 1995) or above the downgoing continental plate adjacent to a collision zone (peripheral or proforeland type; Miall, 1995). The downward fl exure is accompanied by the uplift of a forebulge in the basement, typically a few hundred kilometers out from the fold-thrust belt, and beyond that there is a shallow depression called the backbulge basin. The relationship between crustal loading and basin formation was fi rst articulated by Price (1973) with respect to the foreland basin of southern Alberta, and subsequent modeling of the Alberta Basin by Beaumont (1981) demonstrated the quantitative link between fl exural loading, the formation of a crustal depression, and its fi lling by syntectonic sedimentation. Jordan (1981) demonstrated another key criterion for the defi nition of the foreland basin style of subsidence and sedimentation: the distinctive isopach pattern of the basin fi ll, elongated parallel to the mountain front and asymmetric in cross section, with a depocenter located close to the contemporaneous fold-thrust belt, the uplift of which serves as the proximal sediment source. Cross (1986) used this criterion to distinguish foreland basin subsidence from that due to subcrustal mantle loading, which yields a much broader, wider pattern of basinal subsidence. The Western Interior basin (of which the Alberta basin and the Rocky Mountain basin are parts) was initiated as a retroarc basin as a result of a fi rst-order change in the plate kinematics of Pangea. This supercontinent, assembled by multiple plate convergence and suture between Ordovician and Permian time, began to fragment by rifting in the Triassic, and by early Middle Jurassic time, oceanic crust was forming off what is now the Atlantic coast of the United States. The North American continent began a long process of northwestward drift, which carried the plate some 70° of longitude westward, relative to the Pacifi c plate, and, some 40° of latitude northward until Paleocene time, followed by a change in trajectory and a subsequent southward drift of ~10° up to the present day (Engebretson et al., 1985). There are numerous indicators of a fi rst-order change in magmatic, tectonic subsidence and sedimentation patterns associated with this change in plate kinematics. The fi rst appearance of westerly-derived detritus from a rising arc or orogen has long been accepted as providing the timing of initiation of the Western Interior foreland basin. Prior to the establishment of the basin, the western continental margin was miogeoclinal in character, with sediment derived from easterly sources (the craton and the Canadian Shield). Ron Blakey and I (in Miall, 2008), designated the